1. Field of the Invention
The present invention relates generally to cathode ray tube (CRT) manufacture. More specifically the present invention relates to tensioned-mask color CRT manufacture.
2. Background Art
It is known in the art of CRT manufacture to provide the color CRT with a tensioned shadow mask to direct electron beams to the proper image-producing phosphors on the screen. The tensioned shadow mask offers advantages in CRT power handling capability, conservation of materials, and ruggedization over the familiar free-standing, framed shadow mask.
A current method of phosphor screen deposition is photolithography through a production shadow mask which is then mated to that particular screened panel for incorporation into the working CRT. This led to several tensioned mask CRT designs which seek to secure the tensed mask to a self-supporting mask frame. The tensed mask and frame are used for screen deposition and later incorporated into the operating CRT. Examples include U.S. Pat. No. 4,704,094 to Stempfle; and 4,595,857 to Rowe et al., owned by the assignee hereof. The U.S. Patents cited incorporate the mask and self-supporting frame into the CRT envelope. However, if the self-supporting frame is not sufficiently heavy and rigid it will distort under the forces applied to it by the affixed tensed mask. The mask aperture pattern of the tensed mask will likewise be distorted. This distortion is "transferred" to the screen during photodeposition when utilizing the tensed mask and frame structure as the "photo-template". But, when incorporating the mask and frame structure into the finished tube at elevated temperatures the strain of the tensed mask is relieved, allowing the frame to resume an undistorted shape. The frame is then "frozen" in this undistorted shape upon incorporation into the tube envelope, resulting in an undistorted shadow mask aperture pattern, upon cooling of the tube. Thus, misregistration between the applied screen and the operational tube shadow mask aperture pattern will result. The reader is referred to U.S. Pat. No. 4,686,415, commonly owned herewith, for a further exposition of this phenomenon.
Such "self-supporting" frame designs also negate the possible material saving advantages of tensioned mask design by still requiring a mask frame heavy enough to prevent distortion of the attached mask during screen deposition photolithography.
In order to overcome the problems attendant with incorporating the mask and self-supporting frame into the CRT, the assignee hereof, as. generally disclosed in U.S. Pat. No. 4,737,681; tenses the mask on a separate assembly or frame and uses a shadow mask support affixed directly to the interior surface of the front panel. In this manner the tensed mask support may derive mask-holding strength from the front panel, thereby realizing material and weight savings. Further, this arrangement allows for direct measurement of, and compensation for, the critical mask-to-screen distance, or "Q"-height, while allowing easy access to the mask support surface during manufacture of the CRT.
However, this particular panel-mounted mask support design requires increased usage of the front panel glass area which might otherwise be used for the application of image-forming screen phosphors. This results in an undesirable loss of screen area per unit area of panel glass and overall enlargement of, for example, a television set for a given screen size. Panel glass is also known to be heavy and, in its current individually stamped panel form, expensive.
Relatedly, and as disclosed in U.S. Pat. No. 4,816,053 to Palac, commonly owned herewith; flat tensioned mask CRTs may utilize less expensive float glass for the front panel. However, to date it has been found that float glass is difficult to work with in CRT applications owing to its edge finishing requirements necessitated by its sharp edges upon scoring and breaking, or rough edges upon abrasive cutting. Rough float glass panel edges are believed to contribute to an increased sensitivity to panel breakage during CRT manufacture. Sharp edges are, of course, a safety hazard. Edge finishing requires additional expense.
Thus, in order to obtain the inherent advantages of tensioned mask CRTs there is needed a mask support system which utilizes the strength of other CRT envelope components and provides for maximum screen area per unit area of panel glass. Further, cost advantages of tensed mask CRTs may be realized with a reliable way of using float glass for the front panel.
Other references of possible interest may include U.S. Pat. Nos. 4,593,224; 4,593,225; 4,826,463 and 4,595,857, commonly owned herewith; as well as U.S. Pat. Nos. 4,900,977 and 4,925,421.